The occurrence of lignin as a waste product in the chemical treatment of wood, particularly in the pulp and paper industry, has made it an attractive raw material for adhesives. (Advanced wood adhesives technology. A. Pizzi, Marcel Dekker Inc. New York, Basel, Hong Kong. 1994, p. 289)
Because of the economy of using technical lignin instead of standard urea formaldehyde (UF) and phenol formaldehyde (PF) resins, there are reported numerous efforts to extend these resins with technical lignin (Lignin--Adhesive research for wood composites. Terry Sellers, Jr. Technical Editor. 1995, Mississippi University).
The first patents dealing with the application of spent sulfite liquor (SSL) as adhesive for paper, wood and other lignocellulosic materials date back to the end of the nineteenth century. At the present time, technical lignins include at least the following:
lignosulfonate, sulfate lignin, hydrolysis lignins, biological lignins, explosive lignins and organosolve lignins.
Lignosulfonate (LS) commonly is used as a generic term for spent sulfite liquor (SSL), lignosulfonate purified from carbohydrates, sulfonated alkali lignin from alkaline pulping processes, e.g. kraft process, and sulfonated hydrolysis lignin, obtained from wood saccharification. For the purposes of this invention LS refers to SSL or LS purified from SSL.
There are two ways to utilize lignosulfonate in adhesives; as the adhesive itself, or mixed with UF or PF resins. As is known in the art, LS is unacceptable for use as an adhesive, because of the extremely long curing time, the high curing temperature needed, and the acidity. Also, LS resins require additional treatment after the compression step. Also, the product has a dark color and low physical and mechanical properties and water resistance.
K. C. Shen and L. Calve, Ammonium-based spent sulfite liquorfor waferboard binder, Adhes. Age, 25-29 (Aug. 1980), and in Canadian Patent No. 2,410,746, disclose making particleboard by condensation reaction of LS. Its properties are inferior to those required for exterior grade particleboard made with PF resin. N. H. Nimz and G. Hitze, The application of spent sutfile liquor as an adhesivefor particleboard, Cellul. Chem. Technol. 14: 371-382 (1980) suggested radical polymerization of LS using hydrogen peroxide. In this case, the formation of new carbon--carbon as well as carbon--oxygen bonds between two radicals is a very fast reaction with low activation energy which needs no external heating or strong mineral acid as a catalyst. However, the results have the drawbacks of a large consumption of hydrogen peroxide (9-10%), and a dark color, which is unacceptable to most users.
The second manner is to use lignosulfonate or modified lignosulfonate in a mixture with UF or PF resins. Conventional techniques for modifying or preprocessing lignin into a water-soluble product exist for use as a binder in various wood processes. One prior art technique involves the methylolation of lignin (e.g. sulfite lignin). For example, as described in Lin, U.S. Pat. No. 4,332,589, lignin is methylolated by treatment with formaldehyde under alkaline conditions at a temperature in the range of about 60 to 90 degrees C. The resultant lignin is then acidified to a pH below 7 and heated to a higher elevated temperature. This technique is further set forth in Schmitt et al, U.S. Pat. No. 5,075,402.
K. Forss and G. Agnetta (U.S. Pat. No. 4,105,606) suggested an adhesive for the manufacture of plywood, fiberboard, and particleboard, containing the combination of phenol formaldehyde resin, and lignin derivatives such as lignosulfonates or alkali lignins. According to the invention, a minimum of 65% by weight of lignosulfonates and a minimum 40% of the alkali lignins have relative molecular weights in excess of that of Glucagon, (3483 Daltons).
In Zaslavsky's U.S. Pat. No. 4,276,077, the reagents used are graft polymers obtained from crude lignosulfonate, and a monomer selected from the group consisting of vinyl cyanide (acrylonitrile), vinyl acetate, hydrolyzed vinyl acetate and acrylamide or combinations thereof, at a pH range of between 2 and 6 in the presence of an initiator.
Lin suggested in U.S. Pat. No. 4,332,589 a two-stage treatment of lignin, the first stage carried out with formaldehyde, and the second stage with air or oxygen to increase the molecular weight. The lignin is first treated with from 0.5 to 3.5 moles of formaldehyde per 1000 grams of lignin at a pH between 10.5 and 11.5 and a temperature from 50 degrees C. to 80 degrees C. for from 3 to 24 hours to form a lignin formaldehyde adduct, preferably with minimum crosslinking of lignin. Then the thus formed lignin formaldehyde adduct is oxidized at a temperature of from 25 degrees C. to 80 degrees C. with air or molecular oxygen for from 2 to 24 hours.
U.S. Pat. No. 4,546,173 describes a method of methylolation of sulfonated lignin suitable for use as dispersants and adhesives wherein sulfonated lignins are post sulfonation crosslinked with a crosslinking agent of the aldehyde, epoxide or polyhalide type at pH range of between about 6.1 to 9 to selectively crosslink the low molecular weight lignins to provide improved heat stability and dispersibility of the sulfonated lignins in dye compositions.
Hume et al suggested in U.S. Pat. No. 4,564,649 an aqueous adhesive possessing sufficient adhesion, tack, open time, thermal stability, biological stability, dimensional stability, flexibility, and adhesion contained in an aqueous base of polyvinyl alcohol and lignosulfonate wherein there are about 1 to 8 parts of the lignosulfonate per each part of the polyvinyl alcohol.
Thus in principle it has been shown that technical lignin, in particular SSL or lignosulfonate, can be used to replace aminoplast or phenol formaldehyde resin in quantities of up to 10-15% of the resin without unacceptably affecting the resin quality. But as the quantity of lignin derivatives increases, there is a concurrent increase in the curing time, and temperature, and a decrease in pH, which can cause corrosion of the composition board press plates, an unacceptable drawback. Also, there is a tendency to affect the product's mechanical properties, and increase the emission of free formaldehyde from the urea--formaldehyde resin.